User terminal, radio base station and radio communication method

ABSTRACT

The present invention is designed to improve throughput when CA is executed between multiple cells including a TDD cell. A user terminal carries out radio communication with a plurality of cells by employing carrier aggregation, and has a receiving section that, when a connection is established with a TDD cell, receives information about a predetermined DL/UL configuration that is selected from a plurality of DL/UL configurations, and a control section that controls transmission and reception to and from the TDD cell based on the predetermined DL/UL configuration received, and a DL/UL configuration to carry out DL communication in all subframes is included as one in the plurality of DL/UL configurations, and the control section uses the DL/UL configuration to carry out DL communication in all subframes as one of the DL/UL configurations only when the connecting TDD cell is a secondary cell.

TECHNICAL FIELD

The present invention relates to a user terminal, a radio base stationand a radio communication method that are applicable to anext-generation communication system.

BACKGROUND ART

In the UMTS (Universal Mobile Telecommunications System) network, thespecifications of long term evolution (LTE) have been drafted for thepurpose of further increasing high speed data rates, providing lowerdelay and so on (see non-patent literature 1). In LTE, as multipleaccess schemes, a scheme that is based on OFDMA (Orthogonal FrequencyDivision Multiple Access) is used in downlink channels (downlink), and ascheme that is based on SC-FDMA (Single Carrier Frequency DivisionMultiple Access) is used in uplink channels (uplink). Also, successorsystems of LTE (referred to as, for example, “LTE-advanced” or “LTEenhancement” (hereinafter referred to as “LTE-A”)) have been developedfor the purpose of achieving further broadbandization and increasedspeed beyond LTE, and the specifications thereof have been drafted (Rel.10/11).

As duplex modes for radio communication in the LTE and LTE-A systems,there are frequency division duplex (FDD) to divide between the uplink(UL) and the downlink (DL) based on frequency, and time division duplex(TDD) to divide between the uplink and the downlink based on time (seeFIG. 1A). In TDD, the same frequency region is applied to uplink anddownlink communication, and signals are transmitted and received to andfrom one transmitting/receiving point by dividing between the uplink andthe downlink based on time.

In TDD in the LTE system, a plurality of frame configurations (DL/ULconfigurations) with different transmission ratios of uplink subframes(UL subframes) and downlink subframes (DL subframes) are stipulated. Tobe more specific, as shown in FIG. 2, seven frame configurations, namelyDL/UL configurations 0 to 6, are stipulated, where subframes #0 and #5are allocated to the downlink and subframe #2 is allocated to theuplink.

Also, the system band of the LTE-A system (Rel. 10/11) includes at leastone component carrier (CC), where the system band of the LTE systemconstitutes one unit. Gathering a plurality of component carriers(cells) to achieve a wide band is referred to as “carrier aggregation”(CA).

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: 3GPP TS 36.300 “Evolved UTRA and Evolved    UTRAN Overall Description”

SUMMARY OF INVENTION Technical Problem

Generally speaking, DL traffic and UL traffic are asymmetrical. Also,the ratio between UL traffic and DL traffic is not constant, and variesover time or between locations. When carrier aggregation (CA), which wasintroduced in Rel. 10, is employed, in TDD, geographically-neighboringtransmission points are confined to the use of the same DL/ULconfiguration in a given frequency carrier in order to preventinterference between a plurality of CCs (also referred to as “cells,”“transmitting/receiving points,” etc.). In Rel. 11, CA (TDD inter-bandCA) to employ different DL/UL configurations between different cells issupported, in order to enable flexible switching of DL/UL configurationsin accordance with traffic.

Also, in carrier aggregation (CA) in Rel. 10/11, the duplex modes toapply between a plurality of CCs need to be the same duplex mode (seeFIG. 1 B). On the other hand, future radio communication systems (forexample, Rel. 12 and later versions) may anticipate CA to employdifferent duplex modes (TDD+FDD) between multiple CCs (see FIG. 1C).

In this way, provided that the forms of the use of radio communicationsystems have been growing in variety, there is an even stronger demandfor flexible control of UL communication and DL communication takinginto account traffic and so on. However, when existing mechanisms (forexample, the existing DL/UL configurations in TDD) are used, there is athreat that the throughput cannot be improved sufficiently.

The present invention has been made in view of the above, and it istherefore an object of the present invention to provide a user terminal,a radio base station and a radio communication method which can improvethroughput when CA is executed between multiple cells including at leasta TDD cell.

Solution to Problem

The user terminal of the present invention provides a user terminal thatcarries out radio communication with a plurality of cells by employingcarrier aggregation, and this user terminal has a receiving sectionthat, when a connection is established with a TDD cell, receivesinformation about a predetermined DL/UL configuration that is selectedfrom a plurality of DL/UL configurations, and a control section thatcontrols transmission and reception to and from the TDD cell based onthe predetermined DL/UL configuration received, and a DL/ULconfiguration to carry out DL communication in all subframes is includedas one in the plurality of DL/UL configurations, and the control sectionuses the DL/UL configuration to carry out DL communication in allsubframes as one of the DL/UL configurations only when the connectingTDD cell is a secondary cell.

Advantageous Effects of Invention

According to the present invention, it is possible to improve throughputwhen CA is executed between multiple cells including a TDD cell.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 provide diagrams to explain an overview of duplex modes in LTEand LTE-A, and intra-base station CA (intra-eNB CA);

FIG. 2 is a diagram to explain DL/UL configurations for use in TDD cellsof existing systems;

FIG. 3 is a diagram to show an example of DL/UL configurations for usein TDD cells according to the present embodiment;

FIG. 4 provide diagram to show examples of system structures where a TDDcell (SCell) uses DL/UL configuration 7, according to the presentembodiment;

FIG. 5 is a diagram to show another example of DL/UL configurations foruse in TDD cells according to the present embodiment;

FIG. 6 is a diagram to show an example of A/N feedback when a TDD cell(SCell) uses DL/UL configuration 7, according to the present embodiment;

FIG. 7 is a diagram to show another example of A/N feedback when a TDDcell (SCell) uses DL/UL configuration 7, according to the presentembodiment;

FIG. 8 is a schematic diagram to show an example of a radiocommunication system according to the present embodiment;

FIG. 9 is a diagram to explain an overall structure of a radio basestation according to the present embodiment;

FIG. 10 is a diagram to explain a functional structure of a radio basestation according to the present embodiment;

FIG. 11 is a diagram to explain an overall structure of a user terminalaccording to the present embodiment;

FIG. 12 is a diagram to explain a functional structure of a userterminal according to the present embodiment; and

FIG. 13 is a diagram to show examples of the number of OFDM symbols touse for the PDCCH when DL/UL configuration 7 is employed, according tothe present embodiment.

DESCRIPTION OF EMBODIMENTS

As noted earlier, two duplex modes—namely, FDD and TDD—are stipulated inthe LTE and LTE-A systems (see above FIG. 1A), and, in TDD,communication is carried out between a radio base station and a userterminal by using a predetermined DL/UL configuration that is selectedfrom DL/UL configurations 0 to 6 shown in above FIG. 2. In this way, inTDD, the transmission ratio of DL subframes and UL subframes varies perDL/UL configuration, and, the delivery acknowledgement signal (A/N:Acknowledgement/Negative Acknowledgement) feedback mechanism (HARQmechanism) and so on are stipulated for each configuration.

Also, the Rel. 12 and later systems anticipate CA that employs differentduplex modes (TDD+FDD) between multiple CCs (see above FIG. 1C). Inrelationship to this, a study is in progress to employ DL/ULconfigurations 0 to 6 in TDD in the same manner as in LTE 10. However,when CA is employed between a plurality of cells including a cell thatemploys TDD (hereinafter also referred to as a “TDD cell”), there is athreat that existing DL/UL configurations may have difficulty improvingthroughput.

For example, assume a case where DL traffic is heavier than UL traffic.In this case, it may be possible to select a predetermined cell from aplurality of cells where CA is employed, and use this for DLcommunication. For example, in the event of two-CC CA, it may bepossible to use one CC exclusively for DL communication.

In this case, if the selected cell is a cell to employ FDD (hereinafteralso referred to as an “FDD cell”), DL communication is possible inevery subframe. On the other hand, when the selected cell is a TDD cell,it may be possible to employ the DL/UL configuration in which theconfiguration ratio of DL subframes is the highest (in FIG. 2, DL/ULconfiguration 5).

However, even when DL/UL configuration 5 with the highest DL subframeconfiguration ratio is employed, a UL subframe and a special subframeare included (SF #1 and SF #2). Consequently, when a TDD cell is usedexclusively for DL communication, subframes that cannot be used in DLdata communication are produced. As a result of this, sufficientimprovement of throughput cannot be achieved.

In this way, the present inventors have focused on the fact that it isnot possible to optimize throughput with existing DL/UL configurationswhen CA is executed by using a plurality of cells including a TDD cell,and come up with the idea of using a new DL/UL configuration. To be morespecific, the present inventors have come up with the idea of defining anew, additional DL/UL configuration (for example, DL/UL configuration 7)for DL communication in a TDD cell, and using this DL/UL configuration 7only when the TDD cell serves as a secondary cell (SCell) (not theprimary cell (PCell)).

Furthermore, the present inventors have focused on the fact that a newA/N feedback mechanism (HARQ mechanism) is required when a DL/ULconfiguration for DL communication (for example, DL/UL configuration 7)is use in a TDD cell. To be more specific, the present inventors havecome up with the idea of controlling A/N feedback by seeing a TDD cellthat uses DL/UL configuration 7 as an FDD cell.

Now, a specific radio communication method according to the presentembodiment will be described below in detail with reference theaccompanying drawings. Note that, although cases will be described inthe following description where two cells (two CCs) carry out CA, thenumber of cells that can employ CA according to the present embodimentis by no means limited to this. Also, the present embodiment can beapplied to intra-base station CA (intra-eNB CA), in which a scheduler isprovided to be shared by multiple cells, and inter-base station CA(inter-eNB CA), in which schedulers are provided separately for each ofmultiple cells, as long as at least one TDD cell is included in thecells that employ CA.

First Example

A case will be described with a first example where, in CA to involve aTDD cell (TDD CC), communication is carried out by adding a new DL/ULconfiguration for DL communication.

With the present embodiment, a DL/UL configuration for DL communicationto enable DL communication in all subframes is newly introduced, inaddition to existing DL/UL configurations 0 to 6 for use in TDD. Forexample, DL/UL configuration 7 is added as the DL/UL configuration forDL communication (see FIG. 3). Consequently, the newly added DL/ULconfiguration for DL communication (hereinafter also referred to as“DL/UL configuration 7”) is configured in the TDD band, as DL/ULconfigurations 0 to 6 are.

For the DL/UL configuration for DL communication to enable DLcommunication in all subframes, a structure to make all subframes DLsubframes, as shown in FIG. 3, can be employed. When all subframes aremade DL subframes, it is possible to improve the spectral efficiency ofDL communication even more. Furthermore, by stipulating DL/ULconfiguration 7 for DL communication as being one the DL/ULconfigurations that can be configured in TDD cells, this has littleimpact on the implementation of terminals and frequency allocation, andtherefore DL/UL configuration 7 can be introduced easily. Meanwhile,when DL/UL configuration 7 is applied to a TDD band, UL-DL interference,which is characteristic of TDD, is not produced, so that network (NW)synchronization, inter-device coordination (for example, between userterminals and base stations) and so on can be made unnecessary.

Also, DL/UL configuration 7, which is added anew, can be used only whenthe TDD cell is configured as a secondary cell (SCell), not the primarycell (PCell). This is because a TDD cell to serve as the primary cellneeds to receive UL communication (A/Ns, CQIs, etc.) from userterminals.

Here, the primary cell (PCell) refers to the cell that manages RRCconnection, handover and so on when CA is executed, and is also a cellthat requires UL communication in order to receive data and feedbacksignals from terminals. The primary cell is always configured wheneverCA is executed in the uplink and the downlink. A secondary cell (SCell)refers to another cell that is configured apart from the primary cellwhen CA is employed. A secondary cell may be configured only in thedownlink, or may be configured in the uplink and the downlinksimultaneously.

Also, application of DL/UL configuration 7 in a TDD cell that serves asa secondary cell may be configured as appropriate by a radio basestation (or a higher station and/or the like) depending on traffic andso on. For example, when a TDD cell is configured as a secondary celland there is a large amount of DL data for user terminals, the radiobase station selects DL/UL configuration 7 from a plurality of DL/ULconfigurations 0 to 7, and reports information about the DL/ULconfiguration to use to each user terminal. The information about theDL/UL configuration can be reported through higher layer signaling(broadcast signal, RRC signaling and so on). For the higher layersignaling, for example, system information block 1, radio resourceconfiguration common information elements (RRC common informationelements) and so on may be possible. Alternatively, terminal-specificradio resource configuration signaling may be used.

Meanwhile, it is also possible to operate a TDD cell based on DL/ULconfiguration 7 semi-statically, and allow the radio base station (or ahigher station and/or the like) to configure CA in user terminals asappropriate. For example, it may be possible to build a cellular networkwith an FDD cell or a TDD cell of DL/UL configurations 0 to 6 andprovide a TDD cell of DL/UL configuration 7 in a place where the trafficis heavy, so that the radio base station may configure the TDD cell ofDL/UL configuration 7 as a secondary cell in user terminals that arecapable of CA. Note that the FDD cell or the TDD cell of DL/ULconfigurations 0 to 6 is configured as the primary cell in the userterminals to execute CA then. By this means, it is possible to off-loadthe DL data for the user terminals where CA is configured, in thesecondary cell. The configuration of CA, the configuration of theprimary cell and the secondary cell may be reported through higher layersignaling (broadcast signal, RRC signaling and so on). For the higherlayer signaling, for example, system information block 1, radio resourceconfiguration common information elements (RRC common informationelements) and so on may be possible. Alternatively, terminal-specificradio resource configuration signaling may be used.

Note that, with the present embodiment, the primary cell may be either aTDD cell or an FDD cell, and, when the secondary cell is a TDD cell,DL/UL configuration 7 can be configured in this TDD cell. Also, thepresent embodiment is applicable when a TDD cell or an FDD cell of thesame or a different DL/UL configuration is additionally configured as anSCell and CA is executed in three or more CCs.

FIG. 4 show examples of CA, in which a TDD cell of DL/UL configuration 7is included as a secondary cell. FIG. 4A shows a case where CA isexecuted by employing a duplex mode between multiple CCs (cells) (TDDinter-band CA). The primary cell is a TDD cell of DL/UL configurations 0to 6, and a TDD cell of DL/UL configuration 7 is a secondary cell. FIG.4B shows a case where CA is executed by employing different duplex modesbetween different multiple CCs (cells) (TDD-FDD CA). The primary cell isan FDD cell, and a TDD cell of DL/UL configuration 7 is a secondarycell.

In FIG. 4A, in the first TDD cell that serves as the primary cell, apredetermined DL/UL configuration is selected from existing DL/ULconfigurations 0 to 6 and employed. Meanwhile, in the second TDD cell toserve as a secondary cell, a predetermined DL/UL configuration isselected from existing DL/UL configurations 0 to 6, with an addition ofDL/UL configuration 7, and employed. For example, when the second TDDcell is used exclusively for DL communication, DL/UL configuration 7 isconfigured in the second TDD cell.

In FIG. 4B, in the FDD cell that serves as the primary cell, ULcommunication and DL communication are carried out in every subframe. Onthe other hand, in the TDD cell to serve as a secondary cell, apredetermined DL/UL configuration is selected from existing DL/ULconfigurations 0 to 6 (+DL/UL configuration 7), and employed. Forexample, when the TDD cell is used exclusively for DL communication,DL/UL configuration 7 is configured in the TDD cell.

In this way, by newly defining and using DL/UL configuration 7 for DLcommunication in a TDD cell to serve as a secondary cell, it becomespossible to improve the throughput of DL communication regardless of theduplex mode configured in the primary cell.

Note that, although a case is illustrated in above FIG. 3 where allsubframes (SFs #0 to #9) are made DL subframes, the present embodimentis by no means limited to this. A structure to employ DL/ULconfiguration 7 and provide a special subframe in one of the subframesmay be used. For example, as shown in FIG. 5, a predetermined subframe(here, one SF #1) is configured as a special subframe. In this case, byusing this one special subframe in a TDD cell that serves as a secondarycell, it is possible to transmit the SRS (sounding reference signal) andthe PRACH signal (random access signal), in addition to carrying out DLcommunication.

By enabling transmission of the SRS and the PRACH in this specialsubframe, DL adaptive transmission, including precoding, adaptivemodulation and so on, becomes possible, by using the symmetry ofchannels. In a TDD cell, communication is carried out by using the samefrequency resources and by switching between the uplink and the downlinkover time. Consequently, when terminals/base stations and theirsurrounding environment move only moderately, the channels also varymoderately, so that transmission to utilize the symmetry of channelsbecomes possible. For example, a radio base station may estimate channelstates based on SRSs transmitted from user terminals, and carry outtransmission precoding that is suitable for the estimated channelstates.

Also, in special subframes, not only the SRS, the PRACH and so on, butalso feedback information for a TDD cell where DL/UL configuration 7 isconfigured may be transmitted. For the feedback information, CSI(Channel State Information), which represents the quality of receptionin user terminals, delivery acknowledgement signals in response to DLsignals from the TDD cell and so on may be used. By this means, itbecomes possible to transmit only minimal UL signals, so that it becomespossible to acquire information that is required in HARQ, adaptivemodulation and so on on the radio base station side, without using theprimary cell's UL resources in the TDD cell alone. As a result of this,it is possible to improve the situation where the primary cell's ULresources become short, by configuring the TDD cell of DL/ULconfiguration 7 as a secondary cell.

The feedback information may be linked with the sequences or theresources of the SRS, the PRACH and so on that are transmitted in ULresources (UpPTS) of special subframes, or it is equally possible todefine a new special subframe format, and transmit the feedbackinformation in the PUSCH (Physical Uplink Shared Channel) or the PUCCH(Physical Uplink Control Channel) by means of UpPTS. Also, feedback bymeans of special subframes may be always applied whenever DL/ULconfiguration 7 is configured, or may be configured apart from DL/ULconfiguration 7 by means of broadcast information, RRC signaling and soon (that is, feedback by means of special subframes can be configuredper user terminal that communicates in DL/UL configuration 7). Ifseparate configuration is possible, the radio base station or thenetwork can have control as to which of the primary cell and a secondarycell of DL/UL configuration 7 sends feedback, on a per terminal basis,so that it is possible to change the feedback format flexibly dependingon the conditions and the number of terminals, traffic, the deploymentof cells and so on.

Note that, when there is no need to transmit the SRS and the PRACHsignal in a TDD cell that serves as a secondary cell, it is preferableto use the configurations shown in above FIG. 3 from the perspective ofimproving the spectral efficiency of DL communication and making network(NW) synchronization and inter-device coordination unnecessary.

In this way, when CA is executed using a plurality of cells including aTDD cell, it is possible to effectively improve the spectral efficiencyof DL communication by using DL/UL configuration 7 for DL communicationin the TDD cell that serves as a secondary cell. Also, by defining DL/ULconfiguration 7 for DL communication anew, communication that issuitable for DL traffic and UL traffic become possible in TDD cells.Furthermore, when DL/UL configuration 7 is used in a TDD cell, UL-DLinterference, which is characteristic of TDD, is not produced, so thatnetwork (NW) synchronization, inter-device coordination (coordinationbetween user terminals and base stations) and so on can be madeunnecessary.

<Variation>

Note that, with the above first example, the radio communication method(for example, the DL/UL configuration which each user terminal employs)may be controlled taking into account the legacy terminals that cannotsupport (identify) DL/UL configuration 7 (for example, user terminals ofRel. 11 or earlier versions).

For example, assume a case where a TDD cell to serve as a secondary cellis operated based on DL/UL configuration 7 semi-statically, and a radiobase station (or a higher station and/or the like) configures CA in userterminals as appropriate depending on traffic and so on. In this case, astructure may be used in which the TDD cell is operated based on DL/ULconfiguration 7, and arbitrary DL/UL configurations, including DL/ULconfiguration 7, are reported to user terminals by means ofterminal-specific radio resource configuration signaling.

For example, there is a possibility that a terminal that cannot identifyDL/UL configuration 7 (for example, a Rel. 11 terminal) is present amonga plurality of user terminals. Even in this case, as described above, itis possible to configure a secondary cell with a different DL/ULconfiguration from DL/UL configuration 7 (for example, DL/ULconfiguration 2 or DL/UL configuration 5), for the terminal that isunable to identify DL/UL configuration 7, by using terminal-specificradio resource configuration signaling. Meanwhile, for terminals thatsupport DL/UL configuration 7 (that is, terminals that can identifyDL/UL configuration 7 (for example, terminals of Rel. 12 and laterversions), DL/UL configuration 7 may be configured.

In this way, by configuring DL/UL configurations depending on thecapabilities of user terminals, it is not only possible to configure aTDD cell as a secondary cell of DL/UL configuration 7 in which the DLratio is high for terminals that support DL/UL configuration 7, but alsoto allow terminals that do no support DL/UL configuration 7 to connectwith the TDD cell as a secondary cell.

Furthermore, cases might occur where a user terminal that is unable tosupport DL/UL configuration 7 identifies a TDD cell that operates basedon DL/UL configuration 7 as the primary cell or as a cell that operateson a stand-alone basis (non-CA cell). In this case, there is a threatthat the user terminal tries to transmit uplink signals in DL/ULconfiguration 7, in which no UL subframe is configured (for example,uplink random access and so on). So, the present embodiment may bestructured so that a user terminal that is unable to support DL/ULconfiguration 7 cannot identify a TDD cell that uses DL/UL configuration7 as the primary cell or a non-CA cell. Note that a cell that operateson a stand-alone basis refers to a cell that can establish an initialconnection with a user terminal independently (that is, without being asecondary cell (SCell) in CA).

For example, a TDD cell that operates based on DL/UL configuration 7 maybe structured not to transmit system information blocks. This TDD cellis configured only as a secondary cell, and never used as the primarycell or as a non-CA cell. System information of a cell that isconfigured as a secondary cell can be signaled on a per user terminalbasis, by using radio resource configuration signaling by the primarycell.

Consequently, when a secondary cell is configured, a user terminal doesnot have to receive the system information in that secondary cell. Onthe other hand, when a user terminal connects with a TDD cell as theprimary cell or as a non-CA cell, the user terminal must receive thesystem information in this cell. Consequently, by not transmitting thesystem information, it is possible to remove the possibility that aterminal that is unable to identify DL/UL configuration 7 (for example,a Rel. 11 terminal) tries to connect with the TDD cell as the primarycell or as a non-CA cell. By this means, it is possible to prevent aterminal that is unable to identify DL/UL configuration 7 fromtransmitting uplink signals in the TDD cell.

Alternatively, in a TDD cell that operates based on DL/UL configuration7, identification information of an existing DL/UL configuration (forexample, DL/UL configuration 5) may be transmitted in the systeminformation block. By this means, the TDD cell can operate as theprimary cell or as a non-CA cell where an existing DL/UL configurationis configured when there are only user terminals (for example, Rel. 11terminals) that cannot identify DL/UL configuration 7 in the cell, andactually operate as a secondary cell of DL/UL configuration 7 only whenthere is a user terminal that can identify DL/UL configuration 7.

Also, it is possible to configure information to report an existingDL/UL configuration (for example, DL/UL configuration 5) in this TDDcell's system information block, and command dynamic switching of theDL/UL configuration, depending on traffic, the distribution of userterminals and so on, by using an L1/L2 physical downlink control channel(PDCCH (Physical Downlink Control Channel) or EPDCCH (Enhanced PDCCH)),a MAC (Medium Access Control) control element, and so on. Whether or notto switch the DL/UL configuration dynamically can be reported to userterminals in advance by using user-specific radio resource configurationsignaling. Then, when triggered by the L1/L2 PDCCH (Physical DownlinkControl Channel or the MAC control element, the DL/UL configuration ischanged on a temporary basis based on the DL/UL configuration that isconfigured in the system information. Upon detecting the above trigger,a user terminal changes the DL/UL configuration and communicates until apredetermined period of time is over or until a new command is detected.By this means, it becomes possible to change the DL/UL configurationdynamically, depending on the distribution of user terminals, traffic,and so on in TDD cells.

Second Example

A/N feedback mechanism (HARQ mechanism) for when DL/UL configuration 7for DL communication is employed will be described with a secondexample.

As described above, it is possible to improve throughput by defining andusing DL/UL configuration 7 for DL communication anew in a TDD cell thatserves as a secondary cell. On the other hand, when DL/UL configuration7 for DL communication is employed, there are subframes to carry out ULcommunication (A/N feedback and so on), and how A/Ns should be fed backin response to each DL subframe is the problem.

For example, when inter-band CA is carried out in a first TDD cell and asecond TDD cell, with existing mechanisms, feedback in response to DLsubframes of each TDD cell can be sent by using each cell's ULsubframes. However, in TDD-TDD CA shown in above FIG. 4A, when DL/ULconfiguration 7 is used in the second TDD cell, how to feed back A/Ns,CQIs and so on is the problem because no UL subframe is available forthe DL subframes of the second TDD cell.

Consequently, with the present embodiment, when A/Ns and/or the like arefed back, a TDD cell to use DL/UL configuration 7 is seen as an FDDcell, and the feedback of A/Ns and/or the like is controlled (the HARQmechanism is employed). That is, when a TDD cell to serve as a secondarycell employs DL/UL configuration 7, this TDD cell is seen as an FDDcell, and A/Ns, CQIs and/or the like in response to this TDD cell's DLsubframes are multiplexed on predetermined UL subframes of the primarycell.

For example, assume a case where, as shown in above FIG. 4B, an FDD cellis configured as the primary cell, a TDD cell is configured as asecondary cell and DL/UL configuration 7 is used in the TDD cell. Inthis case, the TDD cell is seen as an FDD cell, and a DL HARQ mechanismfor FDD-FDD CA (for example, the HARQ mechanism of Rel. 10) is employed(see FIG. 6).

For example, when the number of CCs (cells) that employ CA is two, it ispossible to multiplex the A/Ns for each DL subframe of the TDD cell onpredetermined UL subframes of the FDD cell, by using PUCCH format 1bwith channel selection (see FIG. 6). Also, when the number of CCs(cells) to employ CA is three or less, it is possible to multiplex theA/Ns for each DL subframe of the TDD cell on predetermined UL subframesof the FDD cell by using PUCCH format 3.

In this way, when the primary cell is an FDD cell, by seeing a TDD cellwhere DL/UL configuration 7 is configured as an FDD cell, it is possibleto see this case as being the same as 2-DL/I-UL CA by an FDD cell toserve as the primary cell and an FDD cell to serve as a secondary cell.In this case, it is possible to send A/N feedback by using the mechanismof FDD-FDD CA that is already stipulated in Rel. 10. As a result ofthis, it is possible to make it unnecessary to introduce a new DL HARQmechanism for DL/UL configuration 7. Consequently, it becomes possibleto implement CA with existing device structures of terminals and radiobase stations, reduce the increase of cost and enable practical use inthe shortest period of time.

Also, assume a case where, as shown in above FIG. 4A, the first TDD cellis configured as the primary cell, the second TDD cell is configured asa secondary cell and DL/UL configuration 7 is used in the second TDDcell. In this case, the second TDD cell is seen as an FDD cell, and theDL HARQ mechanism of 2-DL/1-UL TDD-FDD CA (for example, the HARQmechanism stipulated in Rel. 12) is employed (see FIG. 7).

FIG. 7 shows a feedback mechanism, whereby, in TDD-FDD CA (the PCell isa TDD cell), the A/Ns in response to all DL subframes of an FDD cell (asecond TDD cell) can be allocated to UL subframes of a TDD cell (a firstTDD cell). To be more specific, in FIG. 7, the first TDD cell (PCell) toserve as the PCell employs the A/N feedback timing of DL/ULconfiguration 2. Also, using DL/UL configuration 2 as a basis(reference), the second TDD cell that serves as an SCell allocates theA/Ns in all DL subframes of the second TDD cell to UL subframes of thefirst TDD cell (DL/UL configuration 2+a).

That is, it is possible to send A/N feedback even in response tosubframes (SFs #2 and #7 of the second TDD cell) where normally UL isconfigured in DL/UL configuration 2. Note that the feedback destinationof the A/Ns of these subframes (SFs #2 and #7) may be, for example, thesame feedback destination as that of neighboring subframes. Note that,although FIG. 7 illustrates a case where DL/UL configuration 2 is usedas a basis for the A/N feedback timings of the second TDD cell that isseen as an FDD cell, the present embodiment is by no means limited tothis. Any feedback mechanism may be applicable as long as the A/Ns inall DL subframes of the second TDD cell can be allocated.

Note that the HARQ mechanism that is applicable to the presentembodiment is by no means limited to that in FIG. 7, and any HARQmechanism that is employed in TDD-FDD CA can be used.

Furthermore, depending on the number of CCs, the DL/UL configuration inthe PCell and so on, cases might occur where the number of A/N bits tofeed back in one UL subframe of the PCell becomes greater than the valuethat can be multiplexed in existing PUCCH formats (PUCCH format 1b withchannel selection and PUCCH format 3). Such cases can be handled bylimiting the A/N feedback in response to predetermined DL subframes ofthe SCell. Alternatively, such cases can also be handled by applying A/Nbundling to the A/Ns in response to predetermined DL subframes of theSCell.

In this way, when the primary cell is a TDD cell, seeing a TDD cell,which serves as a secondary cell and in which DL/UL configuration 7 isconfigured, as an FDD cell makes it possible to see this case as beingthe same as 2-DL/1-UL CA by a TDD cell to serve as the primary cell andan FDD cell to serve as a secondary cell. In this case, it is possibleto send A/N feedback by using the TDD-FDD CA mechanism studied in Rel.12. As a result of this, it is not necessary to introduce a new DL HARQmechanism for DL/UL configuration 7.

<UL HARQ>

Furthermore, when DL/UL configuration 7 is used in a TDD cell thatserves as a secondary cell, how to execute UL retransmission control (ULHARQ) in carrier aggregation is the problem. For example, assume a casewhere, as shown in above FIG. 4B, an FDD cell is configured as theprimary cell, a TDD cell is configured as a secondary cell and DL/ULconfiguration 7 is used in the TDD cell. In this case, how to executePUSCH retransmission control in response to the PHICH (PhysicalHybrid-ARQ Indicator Channel) reported from the radio base stations isthe problem.

According to the present embodiment, the TDD cell (DL/UL configuration7) that serves as a secondary cell is seen as an FDD cell, and ULretransmission control in carrier aggregation is executed. For example,assume a case where a user terminal carries out retransmission inresponse to the PHICH (for example, a NACK) that is reported in the n-thsubframe. Note that an A/N that is reported in the PHICH is equivalentto an A/N in response to the uplink signal (PUSCH signal) in an earliersubframe than the n-th subframe (for example, the (n−4)-th subframe).Based on the PHICH that is reported, the user terminal carries outretransmission in response to the PHICH reported in the n-th subframe,in a predetermined subframe (for example, the (n+4)-th subframe), byusing an UL subframe of an FDD cell that serves as the primary cell.Note that the PHICH can be reported from the primary cell to the userterminal.

In this way, when the primary cell is an FDD cell in TDD-FDD CA, byseeing a TDD cell where DL/UL configuration 7 is configured as an FDDcell, it is possible to see this case as being the same as 2-DL/1-UL CAby an FDD cell to serve as the primary cell and an FDD cell to serve asa secondary cell. In this case, it is possible to send A/N feedback byusing the mechanism of FDD-FDD CA that is already stipulated in Rel. 10.As a result of this, it is possible to make it unnecessary to introducea new DL HARQ mechanism for DL/UL configuration 7. Consequently, itbecomes possible to implement CA with existing device structures ofterminals and radio base stations, reduce the increase of cost andenable practical use in the shortest period of time.

Also, with the present embodiment, it is possible to see a TDD cell(DL/UL configuration 7) that serves as a secondary cell as an existingTDD cell (for example, DL/UL configuration 5), and execute ULretransmission control in carrier aggregation. For example, assume acase where a user terminal carries out retransmission in response to thePHICH (for example, a NACK) that is reported in the n-th subframe. Notethat an A/N that is reported in the PHICH is equivalent to an A/N inresponse to the uplink signal (PUSCH signal) in an earlier subframe thanthe n-th subframe (for example, the (n−6)-th subframe). Based on thePHICH that is reported, the user terminal carries out retransmission inresponse to the PHICH reported in the n-th subframe, in a predeterminedsubframe (for example, the (n+4)-th subframe), by using an UL subframeof an FDD cell that serves as the primary cell. Note that, since thereis only one subframe in which uplink signals can be transmitted in theabove-described example, it then follows that there is only one downlinksubframe of the secondary cell in which the PHICH can be transmitted.Consequently, when uplink signals (for example, the primary cell'suplink signals) to correspond to downlink subframes apart from this areretransmitted, it may be possible to use the PDCCH or the EPDCCH, notthe PHICH. Note that the PHICH can be reported from the primary cell tothe user terminal.

In this way, when the primary cell in TDD-FDD CA is an FDD cell, byapplying the same UL retransmission control as in DL/UL configuration 5to a TDD cell in which DL/UL configuration 7 is configured, the PHICH istransmitted in the same downlink subframes as in DL/UL configuration 5.By this means, it is possible to provide adequate support for userterminals which do not support DL/UL configuration 7 (for example, UEsof Rel. 11 or earlier versions) and in which DL/UL configuration 5 isconfigured. This is because the PDCCH mapping method varies depending onwhether or not the PHICH is present in downlink subframes.

Third Example

As noted earlier, in the DL/UL configuration for DL communication thatenables DL communication in all subframes (DL/UL configuration 7), thereare no UL subframes. Consequently, when DL/UL configuration 7 is used,it is possible to stipulate different operations from those of existingDL/UL configurations 0 to 6. Now, an example of the operation in theevent DL/UL configuration 7 is employed will be described.

<Control Channel Format Indicator>

The control channel format indicator (PCFICH: Physical Control FormatIndicator Channel) reports information about the number of symbols usedfor a downlink control channel (PDCCH) in subframes. For example, whenexisting DL/UL configurations 0 to 6 are used in TDD, the number of OFDMsymbols for the PDCCH is limited to two or less in subframes 1 and 6depending on the number of resource blocks (RBs) to use in the downlink(N^(DL) _(RB)) (see FIG. 13).

For example, the number of OFDM symbols for the PDCCH is configured toone or two in the event of N^(DL) _(RB)>10, and the number of OFDMsymbols for the PDCCH is configured to two in the event of N^(DL)_(RB)≦10. This is because, since, in existing DL/UL configurations forTDD, special subframe are configured in subframes 1 and 6 (in subframe6, special subframes are configured only in part of the DL/ULconfigurations), the number of symbols that can be used for DLcommunication (DwPTS) is smaller than with regular DL subframes.

On the other hand, as shown in above FIG. 3, when DL/UL configuration 7,in which no UL subframe or special subframe is configured, is used, thenumber of symbols for DL communication is not limited in subframes 1 and6. Consequently, when DL/UL configuration 7 is used, unlike DL/ULconfigurations 0 to 6, it is possible to stipulate the number of OFDMsymbols for the PDCCH in subframes 1 and 6, without limiting this to twoor less (see FIG. 13). For example, when DL/UL configuration 7 is used,it is possible to make the number of OFDM symbols for the PDCCH one, twoor three in the event of the number of RBs>10, and make the number ofOFDM symbols for the PDCCH two, three or four in the event of the numberof RBs≦10.

In this way, when DL/UL configuration 7 is employed, a structure may beemployed in which the number of OFDM symbols to use for the PDCCH insubframes 1 and 6 can be configured differently from when DL/ULconfigurations 0 to 6 are employed. By this means, when DL/ULconfiguration 7 is employed in TDD, it is possible to control the PCFICH(the number of OFDM symbols to use for the PDCCH) flexibly depending onDL/UL configurations.

<CSI-RS Mapping>

The CSI-RS (Channel State Information-Reference Signal) is a referencesignal for estimating downlink channel states. When existing DL/ULconfigurations 0 to 6 are used in TDD, a user terminal operates on theassumption that the CSI-RS is not transmitted in special subframes. Thisis because, in special subframes, the number of symbols that can be usedfor DL communication (DwPTS) is small compared to that in regular DLsubframes.

On the other hand, when DL/UL configuration 7 shown in above FIG. 3 isused, special subframes are not configured. So, with the presentembodiment, when DL/UL configurations 0 to 6 for TDD, which do notinclude DL/UL configuration 7, are used, a user terminal performsreceiving operations on the assumption that the CSI-RS is nottransmitted in special subframes. That is, a user terminal to employ TDDassumes that the CSI-RS is not configured in special subframes in DL/ULconfigurations 0 to 6, and, in DL/UL configuration 7, can performreceiving operations without assuming that the transmission of theCSI-RS is limited.

<Aperiodic CSI Feedback>

When a CSI request field for triggering an aperiodic CSI report isincluded in a downlink control channel (PDCCH/EPDCCH), a user terminalthat employs FDD sends a report using the PUSCH a predetermined numberof subframes later. For example, when aperiodic CSI report-triggeringinformation is included in a downlink control channel that is receivedin the n-th subframe, aperiodic CSI is fed back using the PUSCH in the(n+4)-th subframe.

As noted earlier, since there are no UL subframes in DL/UL configuration7, how a user terminal using this DL/UL configuration 7 should send anaperiodic CSI report is the problem. Consequently, the presentembodiment may be structured so that aperiodic CSI feedback iscontrolled in the same manner in user terminals that employ DL/ULconfiguration 7 and user terminals that employ FDD.

For example, assume a case where, as shown in above FIG. 4B, an FDD cellis configured as the primary cell, a TDD cell is configured as asecondary cell and DL/UL configuration 7 is configured in the TDD cell.In this case, if aperiodic CSI report-triggering information is includedin a downlink control channel that is received in the n-th subframe, auser terminal feeds back aperiodic CSI in the PUSCH in the (n+4)-thsubframe of the primary cell (FDD cell).

<HARQ-ACK Repetition Operation>

A radio base station can configure the HARQ-ACK repetition operation inuser terminals by using higher layer signaling. A user terminal in whichthe HARQ-ACK repetition operation repeats transmitting A/Ns withpredetermined parameters. In existing systems, HARQ-ACK repetition isallowed only for user terminals where only one serving cell (FDD cell orTDD cell) is configured. Furthermore, HARQ-ACK repetition is allowedonly when A/N bundling is used. This is to secure coverage by improvingthe quality of reception of A/Ns that is deteriorated due to bundling.

Meanwhile, when DL/UL configuration 7 is used, a similar structure tothat of FDD DL is assumed because there are no UL subframes.Consequently, even in TDD, when DL/UL configuration 7 is used, astructure to support HARQ-ACK repetition can be used and is not limitedto the case of executing A/N bundling. By this means, it is possible toimprove the quality of reception of A/Ns.

(Structure of Radio Communication System)

Now, an example of a radio communication system according to the presentembodiment will be described in detail below.

FIG. 8 is a schematic structure diagram of the radio communicationsystem according to the present embodiment. Note that the radiocommunication system shown in FIG. 8 is a system to incorporate, forexample, the LTE system or SUPER 3G. This radio communication system canadopt carrier aggregation (CA) to group a plurality of fundamentalfrequency blocks (component carriers) into one, where the systembandwidth of the LTE system constitutes one unit. Also, this radiocommunication system may be referred to as “IMT-advanced,” or may bereferred to as “4G” or “FRA (Future Radio Access).”

The radio communication system 1 shown in FIG. 8 includes a radio basestation 11 that forms a macro cell C1, and radio base stations 12 a and12 b that are placed inside the macro cell C1 and form small cells C2,which are narrower than the macro cell C1. Also, user terminals 20 areplaced in the macro cell C1 and in each small cell C2. The userterminals 20 can connect with both the radio base station 11 and theradio base stations 12 (dual connectivity). Also, intra-base station CA(intra-eNB CA) or inter-base station CA (inter-eNB CA) is appliedbetween the radio base station 11 and the radio base stations 12. Also,for CA between the radio base station 11 and the radio base stations 12,TDD-TDD CA, TDD-FDD CA and/or the like can be applied.

Communication between the user terminals 20 and the radio base station11 can be carried out by using a carrier of a relatively low frequencyband (for example, 2 GHz) and a narrow bandwidth (referred to as“existing carrier,” “legacy carrier,” etc.). Meanwhile, between the userterminals 20 and the radio base stations 12, a carrier of a relativelyhigh frequency band (for example, 3.5 GHz and so on) and a widebandwidth may be used, or the same carrier as that used in the radiobase station 11 may be used. A new carrier type (NCT) may be used as thecarrier type between the user terminals 20 and the radio base stations12. Between the radio base station 11 and the radio base stations 12 (orbetween the radio base stations 12), wire connection (optical fiber, theX2 interface and so on) or wireless connection is established.

The radio base station 11 and the radio base stations 12 are eachconnected with a higher station apparatus 30, and are connected with acore network 40 via the higher station apparatus 30. Note that thehigher station apparatus 30 may be, for example, an access gatewayapparatus, a radio network controller (RNC), a mobility managemententity (MME) and so on, but is by no means limited to these. Also, eachradio base station 12 may be connected with the higher station apparatusvia the radio base station 11.

Note that the radio base station 11 is a radio base station having arelatively wide coverage, and may be referred to as an “eNodeB,” a“macro base station,” a “transmitting/receiving point” and so on. Also,the radio base stations 12 are radio base stations having localcoverages, and may be referred to as “small base stations,” “pico basestations,” “femto base stations,” “home eNodeBs,” “micro base stations,”“transmitting/receiving points” and so on. The radio base stations 11and 12 will be hereinafter collectively referred to as “radio basestation 10,” unless specified otherwise. The user terminals 20 areterminals to support various communication schemes such as LTE, LTE-Aand so on, and may be both mobile communication terminals and stationarycommunication terminals.

In the radio communication system, as radio access schemes, OFDMA(Orthogonal Frequency Division Multiple Access) is applied to thedownlink, and SC-FDMA (Single-Carrier Frequency Division MultipleAccess) is applied to the uplink. OFDMA is a multi-carrier transmissionscheme to perform communication by dividing a frequency band into aplurality of narrow frequency bands (subcarriers) and mapping data toeach subcarrier. SC-FDMA is a single-carrier transmission scheme tomitigate interference between terminals by dividing the system band intobands formed with one or continuous resource blocks, per terminal, andallowing a plurality of terminals to use mutually different bands.

Here, communication channels to be used in the radio communicationsystem shown in FIG. 8 will be described. Downlink communicationchannels include a PDSCH (Physical Downlink Shared CHannel), which isused by each user terminal 20 on a shared basis, and downlink L1/L2control channels (PDCCH, PCFICH, PHICH and enhanced PDCCH). User dataand higher control information are communicated by the PDSCH. Schedulinginformation for the PDSCH and the PUSCH and so on are communicated bythe PDCCH (Physical Downlink Control CHannel). The number of OFDMsymbols to use for the PDCCH is communicated by the PCFICH (PhysicalControl Format Indicator CHannel). HARQ ACKs and NACKs in response tothe PUSCH are communicated by the PHICH (Physical Hybrid-ARQ IndicatorCHannel). Also, the scheduling information for the PDSCH and the PUSCHand so on may be communicated by the enhanced PDCCH (EPDCCH) as well.This EPDCCH is frequency-division-multiplexed with the PDSCH (downlinkshared data channel).

Uplink control channels include the PUSCH (Physical Uplink SharedCHannel), which is used by each user terminal 20 on a shared basis as anuplink data channel, and the PUCCH (Physical Uplink Control CHannel),which is an uplink control channel. User data and higher controlinformation are communicated by this PUSCH. Also, by means of the PUCCH,downlink radio quality information (CQI: Channel Quality Indicator),ACKs/NACKs and so on are communicated.

FIG. 9 is a diagram to show an overall structure of a radio base station10 (which may be either a radio base station 11 or 12) according to thepresent embodiment. The radio base station 10 has a plurality oftransmitting/receiving antennas 101 for MIMO communication, amplifyingsections 102, transmitting/receiving sections 103, a baseband signalprocessing section 104, a call processing section 105 and acommunication path interface 106.

User data to be transmitted from the radio base station 10 to a userterminal 20 on the downlink is input from the higher station apparatus30, into the baseband signal processing section 104, via thecommunication path interface 106.

In the baseband signal processing section 104, a PDCP (Packet DataConvergence Protocol) layer process, division and coupling of user data,RLC (Radio Link Control) layer transmission processes such as an RLCretransmission control transmission process, MAC (Medium Access Control)retransmission control, including, for example, an HARQ transmissionprocess, scheduling, transport format selection, channel coding, aninverse fast Fourier transform (IFFT) process and a precoding processare performed, and the result is forwarded to eachtransmitting/receiving section 103. Furthermore, downlink controlchannel signals are also subjected to transmission processes such aschannel coding and an inverse fast Fourier transform, and forwarded toeach transmitting/receiving section 103.

Also, the baseband signal processing section 104 reports, to the userterminal 20, control information for allowing communication in the cell,through higher layer signaling (RRC signaling, broadcast signal and soon). The information to allow communication in the cell includes, forexample, information about the DL/UL configurations used in TDD cells,the uplink or downlink system bandwidth, feedback resource informationand so on. Each transmitting/receiving section 103 converts basebandsignals that are pre-coded and output from the baseband signalprocessing section 104 on a per antenna basis, into a radio frequencyband. The amplifying sections 102 amplify the radio frequency signalshaving been subjected to frequency conversion, and transmit the signalsthrough the transmitting/receiving antennas 101. Thetransmitting/receiving sections 103 function as transmission sections totransmit the information about the DL/UL configurations for use in TDDcells, through higher layer signaling (broadcast signal, RRC signalingand so on).

On the other hand, as for data to be transmitted from the user terminal20 to the radio base station 10 on the uplink, the radio frequencysignals that are received in the transmitting/receiving antennas 101 areeach amplified in the amplifying sections 102, converted into basebandsignals through frequency conversion in each transmitting/receivingsection 103, and input in the baseband signal processing section 104.

In the baseband signal processing section 104, the user data that isincluded in the input baseband signals is subjected to an FFT (FastFourier Transform) process, an IDFT process, error correction decoding,a MAC retransmission control receiving process, and RLC layer and PDCPlayer receiving processes, and the result is forwarded to the higherstation apparatus 30 via the communication path interface 106. The callprocessing section 105 performs call processing such as setting up andreleasing communication channels, manages the state of the radio basestations 10 and manages the radio resources.

FIG. 10 is a diagram to show a principle functional structure of thebaseband signal processing section 104 provided in the radio basestation 10 according to the present embodiment. As shown in FIG. 10, thebaseband signal processing section 104 provided in the radio basestation 10 is comprised at least of a control section 301, a DL signalgenerating section 302, a DL/UL configuration selection section 303, amapping section 304, a UL signal decoding section 305 and a decisionsection 306.

The control section 301 controls the scheduling of downlink user datathat is transmitted in the PDSCH, downlink control information that iscommunicated in the PDCCH and/or the enhanced PDCCH (EPDCCH), downlinkreference signals and so on. Also, the control section 301 controls thescheduling of uplink data that is communicated in the PUSCH, uplinkcontrol information that is communicated in the PUCCH or the PUSCH, anduplink reference signals (allocation control). Information regarding theallocation control of uplink signals (uplink control signals and uplinkuser data) is reported to the user terminal by using a downlink controlsignal (DCI: Downlink Control Information).

To be more specific, the control section 301 controls the allocation ofradio resources with respect to downlink signals and uplink signals,based on command information from the higher station apparatus 30,feedback information from each user terminal 20 and so on. That is, thecontrol section 301 functions as a scheduler. Furthermore, when theradio base station 10 uses TDD, the allocation of downlink signals anduplink signals to each subframe is controlled based on the DL/ULconfiguration for use in TDD, selected in the DL/UL configurationselection section 303.

For example, when DL/UL configuration 7 is configured in a TDD cell thatserves as a secondary cell, the control section 301 carries out DLcommunication to the user terminal in all subframes. Also, in inter-eNBCA, the control section 301 is provided for each of multiple CCsseparately, and, in intra-eNB CA, the control section 301 is provided tobe shared by multiple CCs.

The DL signal generating section 302 generates the downlink controlsignals (PDCCH signal and/or EPDCCH signal), downlink data signals(PDSCH signal) and so on that are determined to be allocated by thecontrol section 301. To be more specific, based on commands from thecontrol section 301, the DL signal generating section 302 generates DLassignments, which report downlink signal allocation information, and ULgrants, which report uplink signal allocation information. Also, the DLsignal generating section 302 generates information about the DL/ULconfiguration selected in the DL/UL configuration selection section 303.

The DL/UL configuration selection section 303 selects the DL/ULconfiguration to use in TDD by taking into account traffic and so on.With the present embodiment, a DL/UL configuration for DL communication(for example, DL/UL configuration 7) is defined anew in a TDD cell (seeabove FIG. 3, FIG. 5, etc.). The DL/UL configuration selection section303 can select DL/UL configuration 7 only when this TDD cell is asecondary cell (SCell). For example, the DL/UL configuration selectionsection 303 selects DL/UL configuration 7 from among a plurality ofDL/UL configurations 0 to 7 when a TDD cell is configured as a secondarycell and there is a large amount of data for user terminals. Note thatthe DL/UL configuration selection section 303 can select a DL/ULconfigurations based on information from the higher station apparatus 30and so on.

The mapping section 304 controls the allocation of the downlink controlsignals and downlink data signals generated in the DL signal generatingsection 302 to radio resources based on commands from the controlsection 301.

The UL signal decoding section 305 decodes the feedback signals(delivery acknowledgement signals, etc.) transmitted from the userterminal through an uplink control channel (PUCCH), and outputs theresults to the control section 301. Also, the UL signal decoding section305 decodes the uplink data signals transmitted from the user terminalin the uplink shared channel (PUSCH) and outputs the results to thedecision section 306. The decision section 306 makes retransmissioncontrol decisions (ACKs/NACKs) based on the decoding results in the ULsignal decoding section 308, and outputs the results to the controlsection 301.

FIG. 11 is a diagram to show an overall structure of the user terminalaccording to the present embodiment. The user terminal 20 has aplurality of transmitting/receiving antennas 201 for MIMO communication,amplifying sections 202, transmitting/receiving sections (receivingsections) 203, a baseband signal processing section 204 and anapplication section 205.

As for downlink data, radio frequency signals that are received in theplurality of transmitting/receiving antennas 201 are each amplified inthe amplifying sections 202, and subjected to frequency conversion andconverted into baseband signals in the transmitting/receiving sections203. The baseband signals are subjected to receiving processes such asan FFT process, error correction decoding and retransmission control, inthe baseband signal processing section 204. In this downlink data,downlink user data is transferred to the application section 205. Theapplication section 205 performs processes related to higher layersabove the physical layer and the MAC layer. Also, in the downlink data,broadcast information is also transferred to the application section205. When the user terminal 20 connects with a TDD cell, thetransmitting/receiving sections 203 function as receiving sections thatreceive information about a predetermined DL/UL configuration that isselected from a plurality of DL/UL configurations.

Meanwhile, uplink user data is input from the application section 205 tothe baseband signal processing section 204. In the baseband signalprocessing section 204, a retransmission control (H-ARQ (Hybrid ARQ))transmission process, channel coding, precoding, a DFT process, an IFFTprocess and so on are performed, and the result is forwarded to eachtransmitting/receiving section 203. Baseband signals that are outputfrom the baseband signal processing section 204 are converted into aradio frequency band in the transmitting/receiving sections 203. Afterthat, the amplifying sections 202 amplify the radio frequency signalshaving been subjected to frequency conversion, and transmit the resultsfrom the transmitting/receiving antennas 201.

FIG. 12 is a diagram to show a principle functional structure of thebaseband signal processing section 204 provided in the user terminal 20.As shown in FIG. 12, the baseband signal processing section 204 providedin the user terminal 20 is comprised at least of a DL signal decodingsection 401, a DL/UL configuration determining section 402, a decisionsection 403, a control section 404, a UL signal generating section 405and a mapping section 406.

The DL signal decoding section 401 decodes the downlink control signals(PDCCH signal) transmitted in a downlink control channel (PDCCH), andoutputs the scheduling information (information regarding the allocationto uplink resources) to the control section 404. Furthermore, the DLsignal decoding section 401 decodes the downlink data signalstransmitted in a downlink shared channel (PDSCH) and outputs the resultsto the decision section 403. The decision section 403 makesretransmission control decisions (ACKs/NACKs) based on the decodingresults in the DL signal decoding section 401, and also outputs theresults to the control section 404.

The DL/UL configuration determining section 402 identifies the DL/ULconfiguration-related information that is reported from the radio basestation. The DL/UL configuration determining section 402 outputs theDL/UL configuration-related information that is detected, to the controlsection 404 and so on. Note that, with the present embodiment, the DL/ULconfiguration determining section 402 detects above DL/UL configuration7 only when the user terminal connects with a TDD cell that serves as asecondary cell.

The control section 404 controls the generation of uplink controlsignals (A/N signals and so on) and uplink data signals based on thedownlink control signals (PDCCH signals) transmitted from the radio basestations, retransmission control decisions with respect to the PDSCHsignals received, and so on. The downlink control signals are outputfrom the DL signal decoding section 401, and the retransmission controldecisions are output from the decision section 403.

Also, the control section 404 controls the transmission of uplinkcontrol signals and uplink data signals based on the DL/ULconfiguration-related information that is output from the DL/ULconfiguration determining section 402. For the DL/UL configurations, oneof the DL/UL configurations 0 to 7 shown in above FIG. 3 and FIG. 5 isused. Note that DL/UL configuration 7 is used only when the connectingTDD cell is a secondary cell.

Also, the control section 404 also functions as a feedback controlsection that controls the feedback of delivery acknowledgement signals(A/Ns) in response to the PDSCH signal. To be more specific, in acommunication system in which CA is employed, the control section 401controls the selection of the cells (or the CCs) where deliveryacknowledgement signals are fed back, the PUCCH resources to allocatethe delivery acknowledgement signals to, and so on.

For example, when the primary cell is an FDD cell, a secondary cell is aTDD cell and DL/UL configuration 7 is configured in the TDD cell, thecontrol section 404 can see the TDD cell as an FDD cell, and employ theA/N feedback mechanism in FDD cell—FDD cell CA (see above FIG. 6). Inthis case, the control section 404 feeds back the A/Ns in response toeach DL subframe of the TDD cell by using predetermined UL subframes ofthe FDD cell.

Also, when the primary cell and a secondary cell are TDD cells and DL/ULconfiguration 7 is configured in the TDD cell serving as a secondarycell, the control section 404 can see the TDD cell being a secondarycell as an FDD cell, and employ the A/N feedback mechanism in TDDcell-FDD cell CA (see above FIG. 7). In this case, the control section404 controls A/N feedback so that the A/Ns in response to all DLsubframes of the TDD cell that serves as a secondary cell can beallocated to UL subframes of the primary cell.

The UL signal generating section 405 generates uplink control signals(feedback signals such as delivery acknowledgement signals, channelstate information (CSI) and so on) based on commands from the controlsection 404. Also, the UL signal generating section 405 generates uplinkdata signals based on commands from the control section 404. Note that,when DL/UL configuration 7 is configured, the UL signal generatingsection 405 generates uplink control signals in response to DL signals,without generating uplink data signals.

The mapping section 406 (allocation section) controls the allocation ofuplink control signals (delivery acknowledgement signals and so on) anduplink data signals to radio resources (PUCCH and PUSCH) based oncommands from the control section 404. For example, the mapping section406 allocates the uplink control signals based on the number of CCs, byusing PUCCH format 1b with channel selection, PUCCH format 3 and so on.

Now, although the present invention has been described in detail withreference to the above embodiment, it should be obvious to a personskilled in the art that the present invention is by no means limited tothe embodiment described herein. The present invention can beimplemented with various corrections and in various modifications,without departing from the spirit and scope of the present inventiondefined by the recitations of claims. For example, a plurality ofexamples described above may be combined and implemented as appropriate.Consequently, the description herein is provided only for the purpose ofexplaining examples, and should by no means be construed to limit thepresent invention in any way.

The disclosures of Japanese Patent Application No. 2013-244047, filed onNov. 26, 2013, Japanese Patent Application No. 2014-021429, filed onFeb. 6, 2014 and Japanese Patent Application No. 2014-033562, filed onFeb. 25, 2014, including the specifications, drawings and abstracts, areincorporated herein by reference in their entirety.

1. A user terminal that carries out radio communication with a pluralityof cells by employing carrier aggregation, the user terminal comprising:a receiving section that, when a connection is established with a TDDcell, receives information about a predetermined DL/UL configurationthat is selected from a plurality of DL/UL configurations; and a controlsection that controls transmission and reception to and from the TDDcell based on the predetermined DL/UL configuration, wherein a DL/ULconfiguration to carry out DL communication in all subframes is includedas one in the plurality of DL/UL configurations, and the control sectionuses the DL/UL configuration to carry out DL communication in allsubframes as one of the DL/UL configurations only when the connectingTDD cell is a secondary cell.
 2. The user terminal according to claim 1,wherein the DL/UL configuration to carry out DL communication in allsubframes is comprised entirely of DL subframes.
 3. The user terminalaccording to claim 1, wherein a special subframe is included in theDL/UL configuration to carry out DL communication in all subframes. 4.The user terminal according to claim 1, wherein, when a primary cell isan FDD cell, a secondary cell is a TDD cell and the DL/UL configurationto carry out DL communication in all subframes is configured in the TDDcell, the control section sees the TDD cell as an FDD cell, and employsan A/N feedback mechanism in FDD cell-FDD cell carrier aggregation. 5.The user terminal according to claim 4, wherein the control sectionfeeds back an A/N in response to each DL subframe of the TDD cell byusing a predetermined UL subframe of the FDD cell.
 6. The user terminalaccording to claim 1, wherein, when a primary cell and a secondary cellare TDD cells and the DL/UL configuration to carry out DL communicationin all subframes is configured in the TDD cell that serves as thesecondary cell, the control section sees the TDD cell to serve as thesecondary cell as an FDD cell, and employs an A/N feedback mechanism inTDD cell-FDD cell carrier aggregation.
 7. The user terminal according toclaim 6, wherein the control section controls A/N feedback so that A/Nsin response to all DL subframes of the TDD cell that serves as thesecondary cell can be allocated to UL subframe of the primary cell.
 8. Aradio base station that communicates with a user terminal by employingcarrier aggregation, the radio base station comprising: a DL/ULconfiguration selection that, when a connection is established with aTDD cell, selects a predetermined DL/UL configuration, which the userterminal uses to communicate with the TDD cell, from a plurality ofDL/UL configurations; a control section that controls transmission andreception to and from the user terminal based on the predetermined DL/ULconfiguration, wherein a DL/UL configuration to carry out DLcommunication in all subframes is included as one in the plurality ofDL/UL configurations, and the DL/UL configuration selection section canselect the DL/UL configuration to carry out DL communication in allsubframes only when a secondary cell is configured.
 9. The radio basestation according to claim 8, further comprising a transmission sectionthat reports the predetermined DL/UL configuration to the user terminal.10. A radio communication method for a user terminal that communicateswith a plurality of cells employing carrier aggregation, the radiocommunication method comprising the steps of: when a connection isestablished with a TDD cell, receiving information about a predeterminedDL/UL configuration that is selected from a plurality of DL/ULconfigurations; and controlling transmission and reception to and fromthe TDD cell based on the predetermined DL/UL configuration, wherein aDL/UL configuration to carry out DL communication in all subframes isincluded as one in the plurality of DL/UL configurations, and the DL/ULconfiguration to carry out DL communication in all subframes is used asone of the DL/UL configurations only when the connecting TDD cell is asecondary cell.